70 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



are intermingled with siliceous, bituminous, or other material 

 which may be melted under the influence of heat. This 

 suggested to Hutton his theory that at a certain depth the 

 sedimentary deposits are melted by the heat to which they are 

 subjected, but that the tremendous weight of the super- 

 incumbent water causes the mineral elements to consolidate 

 once more into coherent rock-masses. He applied this theory 

 ]of the melting and subsequent consolidation of rock-material 

 {universally, to all pelagic and terrestrial sediments. 



In the third part it is shown that the present land-areas of 

 the globe are composed of rock-strata which have consolidated 

 during past ages in the bed of the ocean. These are said to 

 have been pushed upward by the expansive force of heat, 

 while the strata have been bent and tilted during the 

 upheaval. Hutton next describes the occurrence of crust- 

 fissures both during the consolidation of the rock and during 

 the elevation of large areas, and the subsequent inrush of 

 molten rock or mineral ores into the fissures. He regards 

 volcanoes as safety-valves during upheaval, which by affording 

 exit at the surface for the molten rock-magma and superheated 

 vapours prevent the expansive forces from raising the con- 

 tinents too far. 



The evidences of volcanic eruption in the older geological 

 epochs are next discussed. Hutton expresses the opinion 

 that during the earlier eruptions the molten rock-material 

 spread out between the accumulated sediments or filled crust- 

 fissures, but did not actually escape at the ^surface ; con- 

 sequently, that the older rock-magmas had solidified at great 

 depths in the crust and under enormous pressure of 

 superincumbent rocks. He calls the older eruptive rocks 

 "subterraneous lavas" and includes amongst them porphyry 

 and the whinstones (eq. trap-rock, greenstone, basalt, wacke, 

 amygdaloidal rocks) ; granite was also added in a later treatise. 

 Hutton points out that the subterraneous lavas have a 

 crystalline structure, whereas those that solidify at the 

 surface have a slaggy or vesicular structure. 



In the fourth part, Hutton concentrates attention on the 

 pre-existence of older continents and islands from which the 

 materials composing more recent land areas must have been 

 derived. He likewise discusses the evidences of pre-existing 

 pelagic, littoral, and terrestrial faunas from which existing 

 faunas must have sprung. But, he continues, the existence of 



